The given error message is defined in the module Online Event Ticketing (technical name website_event_sale).

If you don't mean to use it you can uninstall the module.

But, you should be aware about the fact, that uninstalling modules certainly will have impacts on your database. These impacts are:

• data stored in fields defined by this module only will be removed
• uninstalling website_event_sale may uninstall other modules as well
  You can identify other module that may be removed activating the Show All option in the wizard that is displayed upon uninstalling a module:
  

I would like to strongly advice you to 

1. take a backup prior to uninstalling the module
2. test the uninstalling in a test environment if possible prior to performing the action in your production environment